Acceptable Noise Level as a Predictor of Hearing Aid Use
Nabelek, A.K., Freyaldenhoven, M.C., Tampas, J.W., Burchfield, S.B. and Muenchen, R.A. (2006)
This editorial discusses the clinical implications of an independent research study. The original work was not associated with Starkey Laboratories and does not reflect the opinions of the authors.
A common complaint of hearing aid users is difficulty understanding speech in the presence of background noise. This has led to recent interest in examining acceptable noise levels in hearing aid users. Previous research has not shown a correlation between measures of speech perception in noise and successful hearing aid use (Bentler et. al., 1993; Humes et al., 1996), nor have outcome measures generally been useful to predict success with hearing aids (Schum, 1999). The authors of the current study sought to determine if acceptable noise level measurements can be used to predict success with hearing aids.
Acceptable noise level (ANL), is defined as the difference between the most comfortable listening level for running speech and the maximum background noise level that a listener is willing to accept (Nabalek et al., 1991). Therefore, a lower ANL indicates a better tolerance of background noise. Nabelek and her associates compared full-time, part-time and nonusers of hearing aids to examine the relationship between ANL scores, results from the Speech In Noise test (SPIN) scores (Bilger et. al., 1984) and hearing aid use patterns. One of their primary goals was to investigate whether ANL scores could be used clinically to predict future success with hearing aids.
The authors recruited 191 subjects with sensorineural hearing loss whose audiometric thresholds, on average, had a mild-sloping-to-severe configuration. All subjects were binaural hearing aid users, having been fitted between three months and three years prior to testing. Subjects were fitted by a variety of audiologists who were independent of the study, so the hearing aid models, features and signal processing strategies differed among the subjects.
ANL scores and SPIN scores were obtained with speech and noise presented through the same speaker at a 0° azimuth angle. Following sound field testing, subjects responded to a brief questionnaire, which assigned them to one of three groups of hearing aid users: full-time, part-time and non-users. Full time users were defined as those who wore their hearing aids whenever they needed them, part-time users wore their hearing aids occasionally, and non-users had completely stopped wearing their hearing aids.
The first 58 subjects were tested in three sessions over the first three months of acclimatization with hearing aids. These subjects showed consistent ANL and SPIN scores over the three sessions, so the authors concluded that acclimatization to new hearing aids did not affect test results and remaining subjects were tested in one session only.
The authors found that SPIN and ANL scores were generally not affected by age, gender or pure tone average. SPIN scores were significantly better in the aided condition for all listener groups, regardless of hearing aid use pattern. ANL scores were not significantly different between aided and unaided conditions, but unaided and aided ANLs were strongly correlated to hours of hearing aid use and overall hearing aid use pattern. Full-time hearing aid users demonstrated significantly lower ANL scores than part time users, who in turn demonstrated lower ANL scores than non-hearing aid users. SPIN scores and ANL scores were not strongly correlated. The authors underscored the difference between SPIN and ANL scores, in that SPIN scores indicated the benefit of amplification for speech perception, whereas the ANL scores indicated the difference between successful and unsuccessful hearing aid users. This interpretation of the ANL results was supported by regression analyses that suggest the ANL predicted successful hearing aid outcome with 87% accuracy and unsuccessful hearing aid outcome with 83.6% accuracy. Listeners with ANLs of 5 or below were expected to be successful users and those with ANLs of 15 or higher were expected to be unsuccessful.
Some results of this study should be interpreted with caution for a number of reasons. First, the subjects in the study were all experienced hearing aid users–defined as no more than three years of prior hearing aid use. The authors acknowledged that to truly determine predictive value, unaided testing should be conducted prior to hearing aid fitting, but did not specify that it should be done prior to a first time hearing aid fitting. Therefore, results obtained in the current study inherently represent previous experience with amplification and previously established hearing aid use patterns.
Second, the authors defined full-time hearing aid users as those subjects who used their hearing aids “whenever they needed them”. Many audiologists would question whether this use pattern should truly be considered “full time”. Indeed, clinical experience indicates that patients who use their hearing aids most of the day, every day, are the most successful and comfortable with their hearing aids. Most practicing audiologists would consider patients who use their hearing aids “as needed” to be part-time users, unless “as needed” was defined as a significant portion of every day.
Third, the subjects in this study were fitted by their own audiologists in independent clinics, and more importantly, used a wide variety of hearing instruments with different features and signal processing characteristics. Therefore, their responses to background noise likely varied with signal processing features that were not controlled variables in this study.
Additional work, with strict controls, should be considered in order to truly investigate the predictive value of the ANL test. Future studies should evaluate new hearing aid users prior to their initial fitting in the unaided condition, then conduct subsequent aided testing periodically during an acclimatization period. Under those circumstances, the unaided ANL could not represent longitudinal effects that have already occurred in response to regular hearing aid use. While Nabelek and colleagues (2004) found that there was no change in ANL or SPIN scores for hearing aid users over the first three months of acclimatization. The participant’s previous experience with hearing aids was not controlled; approximately half of the subjects were new users and half had experience with hearing aids. Other studies have shown (Keidser et al., 2008) that acclimatization to hearing aids can extend long beyond the first three months and may continue even beyond one year of use, so the subjective judgments of experienced users could be different from those of inexperienced hearing aid users, even those tested after a few months of use. For this reason, longitudinal study of acceptable noise levels in hearing aid users may be of interest.
One may caution against the use of the ANL as an absolute predictive measure of success with hearing aids. The goal of the clinical audiologist is to rehabilitate hearing impaired individuals, which usually includes fitting them with appropriate hearing aids, counseling and training. If patients who demonstrate high ANL scores are not expected to be successful with hearing aids; should it follow that these patients be discouraged from trying hearing aids? The authors comment that these patients should be counseled about the “limitations of hearing aids even in quiet listening situations”. This conclusion could be detrimental to patient care if hearing aid use is ruled out before the patient is even given a chance to have a trial with them. Regarding subjects with moderate ANL scores, the authors rightly point out the importance of directional microphones and noise reduction, both of which can significantly improve measured ANLs (Pisa et al, 2010). However, these features can be beneficial to all hearing aid users to some degree and would therefore be discussed and recommended regardless of ANL outcome. Perhaps a moderate to high ANL score could indicate a need for more aggressive noise reduction at the initial fitting. This is, however, speculation as no work has been published evaluating the effect of changing noise reduction parameters on ANL scores.
The authors considered their analyses to be an indication that ANL score can predict future success, or at least consistency of use, with hearing aids. However, all of their subjects had previous experience with hearing aids and many had previously established use patterns. Correlation does not implicitly suggest causation; in that regard it should be considered that subjects’ use patterns were predictive of their ANL scores, rather than vice versa. Studies of auditory plasticity have shown that auditory experiences, including use of amplification, can affect objective performance and subjective assessments beyond 12 months after the initial fitting (Palmer et. al., 1998; Keidser et al, 2008). Therefore, it follows that the subjective acceptance of background noise, as measured by the ANL, can differ between new and experienced users and that changes within the auditory system could result in improved noise tolerance.
Perhaps the most important insight to be gained from the current study is the importance of consistent hearing aid use. The authors found that consistent pattern of use of hearing aids was highly correlated to low ANL scores. Rather than supporting the clinical use of ANL scores as a predictive tool, these results may instead indicate that consistent hearing aid users have adjusted to amplified sound and have learned to parse complex acoustic information so that they are better able to withstand increasing background noise levels. All hearing aid patients should be counseled about the importance of consistent use, not only to determine the need for future programming modifications, but also to aid their process of acclimatization to amplified sound, including their acceptance of background noise.
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